It is known in the art to operate a discharge lamp using an open-loop lamp driver circuit. The lamp driver circuit comprises an inverter circuit for generating a suitable AC current for driving the lamp. Such an open-loop driver circuit may be calibrated during manufacturing with respect to the output power.
A known discharge lamp, e.g. an inductively coupled discharge lamp such as a molecular radiation lamp, may exhibit a steep relation between an output power and a voltage over the lamp terminals. The lamp voltage depends, inter alia, on a frequency of the supplied AC current, the output power thereby being depended on the frequency of the supplied AC current. Further, during run-up the impedance of the lamp may exhibit steep changes. Thus, an open-loop lamp driver circuit may not be suitable for driving such a discharge lamp, since the open-loop lamp driver circuit cannot ensure stable operation of the lamp.
Further, it may be desirable to control the lamp power during run-up and steady-state operation. Due to the above-mentioned steep relations, an open-loop lamp driver circuit may not be suitable for regulating the output power.
It is known to use a feedback circuit, and thus a closed-loop lamp driver circuit for driving a discharge lamp. For example, the frequency of the AC current may be controlled in response to an actual lamp power. However, due to EMI regulations, the frequency range for control may be limited, not allowing both controlling stability and regulating power, in particular not during run-up and for dimming.
Another possibility is to control the DC voltage from which the AC current is generated by the inverter circuit. However, due to the presence of a relative large capacitance for energy buffering at the DC-voltage bus, such a control system is relatively slow, whereas a relatively fast control is required for stability control.